It is well known in the automobile industry that improved fuel economy for a vehicle driveline can be achieved by using relatively small displacement internal combustion engines. The performance of undersize displacement engine, however, may be unacceptable for high torque demand driving conditions for the vehicle driveline. It is common practice to improve the performance by providing additional combustion air to the engine intake manifold. This approach will make it possible to enjoy the improved fuel economy associated with smaller displacement engines during low torque demand conditions while providing a reserve torque capability under high performance driving conditions. The additional combustion air supply is provided by an air pump driven by the engine.
It also is common practice with automotive vehicle engines to provide a catalytic converter in the engine exhaust system for the purpose of oxidizing unburned hydrocarbons and oxides of nitrogen to reduce the percentage of undesirable exhaust emissions in the engine exhaust gases. As in the case of the intake manifold boost pressure pump, the air supply for the catalytic converter is developed by an engine-driven air pump.
There are two designs for intake air pressure boost systems known in the prior art that use an air pump for supplying air to the engine intake manifold. A first design employs a positive torque transfer drive from the engine crankshaft to the air pump torque input shaft. A second design uses engine exhaust gas flow to drive a turbine that in turn is mechanically coupled to the torque input shaft of the air pump. The first design is referred to as a supercharged engine system, and the second is referred to as a turbocharged engine system.
The turbocharged engine system, which is used in contemporary automotive vehicle-drivelines more often than the supercharged system, provides an increased volume of intake air for the engine intake manifold. The increased volume of air is mixed with additional fuel, thereby permitting an undersize engine to develop boosted torque output beyond the torque output that would be available with a naturally aspirated carburetor system or with a conventional fuel injection system. Such turbocharged engines thus will exhibit improved fuel economy when the torque requirements are low or moderate and improved engine performance when higher torque is demanded by raising the intake manifold air induction pressure by turbocharging. The turbocharger pump is driven by the turbine torque developed by the exhaust gas flow.
Prior art vehicle engine systems employing a turbocharger in this fashion additionally may require a separate air pump for supplying air to the catalytic converter to reduce exhaust pollutants during engine idle and during vehicle deceleration modes. Such air pumps are referred to as thermactor pumps.
An engine system having a turbocharger air induction system and a thermactor air pump typically exhibits delayed power boost when the vehicle is accelerating from a low speed. It has low engine torque at low engine speeds, although the torque output at high engine speeds is greater than a non-supercharged engine of comparable size. The delayed boost and the low end torque deficiency for the engine are due to the low rate of exhaust gas flow at low engine rpm. Efficient torque output at high engine rpm is due to the increase in exhaust gas flow which results in a driving torque on the turbocharger drive turbine that increases exponentially with engine rpm increases.
In the case of supercharged engines, the engine speed-torque characteristic is more responsive at low engine rpm to driver torque demands. Such supercharged engines, however, exhibit lower fuel economy over the full engine speed range compared to turbocharged engine systems. In a supercharged engine system, the air pump drive is a positive drive rather than a turbine drive, the positive drive being established by gearing or by a belt and pulley coupled to the engine crankshaft. The size of the gears in a geared drive, or the pitch diameter of the pulleys in the case of a belt drive, determines the drive ratio, which is a fixed ratio over all of the engine rpm range. The air pump for the supercharger system then would be driven at a speed that is directly proportional to the engine speed. As in the case of the turbocharged engine system described above, the supercharged engine system usually requires a separate small air pump for the purpose of developing combustion air for the thermactor function.
We are aware of other engine systems having a supercharger and a thermactor wherein a single air pump is used for both supercharging and for developing air for the catalytic converter. Examples of such systems using a single air pump for dual purposes is described in U.S. Pat. No. 4,488,400. The system described in the '400 patent includes a single air pump, but the pump is provided with two pumping chambers. Under certain driving conditions, the outlet side of each of the pumping chambers is distributed to the engine intake manifold. Under other driving conditions, a valve system for controlling the pump responds to signals from sensors that detect engine driving condition variables to deliver air to the catalytic converter. Under still other conditions, the valve system associated with the air pump will direct output air to both the catalytic converter and the engine intake. The speed of the air pump, as in the case of the prior art system described above, is directly proportional to the engine crankshaft speed.
Since a supercharged engine, in contrast to the turbocharger engine system, will provide improved low-end torque and less boost delay, it is common practice for the automotive designer to establish the drive ratio for the air pump drive at a relatively high value. That relatively high ratio, however, causes more air to be delivered to the engine intake and may allow the engine to develop excess torque when the engine speed increases. Such excess high-end torque could result in damage to the engine and to the driveline. It also could result in reduced fuel economy over the entire engine operating range. To prevent overboosting of the engine and overstressing of the driveline, a design compromise must be made as the fixed air pump drive ratio is selected and the air pump capacity is selected in order to avoid excess boost at high engine rpm as an attempt is made to improve the fuel economy at low engine rpm. Thus, the compromise results in less than optimum torque characteristics at both low speeds and at high speeds. The overall fuel efficiency for such a compromised system also is less than optimum.
One design solution that is known in the prior art involves the use of an electromagnetic clutch in the air pump drive system so that the air pump can be disengaged from the engine crankshaft under normal driving conditions. The clutch then can be engaged only during high torque demand conditions. Thus, the air pump does not result in undesirable parasitic losses during normal unboosted driving conditions. The clutch durability and the cost of the clutch system, as well as noise, vibration and harshness problems with such a combustion air boost system, are inappropriate for many vehicle applications. This design approach is described, for example, in prior art U.S. Pat. No. 4,350,135.
Still another design approach known in the prior art involves the use of a bypass valve activated in response to engine vacuum pressure changes. Under normal driving conditions, the bypass valve, which is connected between the inlet port and the outlet port of the air pump, is open so that minimal air pressure is developed by the pump when the pump is rotated. When the pump does not produce pressure, it takes less energy from the engine. The parasitic horsepower loss thus is reduced. This reduction in parasitic loss, of course, improves fuel economy. Under high torque demand conditions, the bypass valve can be closed, thereby permitting the air pump to produce sufficient air volume and pressure to boost the engine intake manifold pressure. The closing of the bypass valve is in response to a reduction in engine vacuum pressure caused by opening the engine throttle as the operator demands higher torque. In many engine applications, however, this design approach is inappropriate because of the durability problems with the air pump resulting from the continuous operation of the air pump at high speeds, even when the pump is not being used to pump air.
Yet another known design approach involves the use of an electric drive for the air pump in a boosted engine system. The air pump then can be driven entirely independently of engine speed. It can be designed, therefore, to be driven at a speed that is desirable for optimum engine boost throughout the entire engine speed range. It has been found, however, that electric drives of this kind are inefficient due to inefficiency in transferring mechanical energy to electrical energy with an alternator. Electrical energy transformation to stored chemical energy with a battery also is a source of inefficiency. The stored chemical energy further must be transferred to electrical energy in the battery; and finally, the electrical energy developed by the battery must be transformed to mechanical energy with a motor. The overall efficiency in a drive of this kind is typically less than 50 percent for most vehicle applications. Furthermore, the alternator size and battery capacity must be increased to accommodate an electric drive, thereby further adding to size, weight and cost to the powertrain.
The inefficiency of an electric drive, in the final analysis, results in a reduced engine fuel economy compared to a pure mechanical drive for the air pump in an engine of comparable size. Although an electric drive for the air pump may result in better performance at the low engine speed range, it will result in reduced engine fuel economy compared to a mechanical supercharger drive throughout the engine speed range.